The invention relates to a semiconductor device and, more particularly to a photo mask that can improve exposure resolution by controlling the transmittance of the photo mask, and to a method for fabricating the same.
In general, a semiconductor memory device, for example a dynamic random access memory (DRAM) device is made of numerous fine patterns, and such fine patterns are formed by a photo lithography process. In the photo lithography process a photoresist film is coated on a target film to be patterned, and exposure and development processes are carried out to form a photoresist film pattern that exposes a partial surface of the target film. Then, an etching process is carried out using this photoresist film pattern as a mask to remove the exposed portion of the target film. Subsequently, tile photoresist film pattern is stripped to form patterns.
Meanwhile, with the decrease in the design rules for devices to be highly integrated, an asymmetric illuminating system, such as a dipole illuminating system is used instead of using a symmetric illuminating system to improve the critical dimension (CD) uniformity and process margin of the patterns. Especially, the dipole illuminating system improves the process margin by carrying out higher resolution exposure to the pattern in the cell region.
FIG. 1 is a diagram illustrating a dipole illuminating system.
Referring to FIG. 1, the exposure process using the dipole illuminating system is carried out to improve resolution of patterns to be formed in the cell region via two apertures formed on the illuminating system. Here, the apertures formed on the illuminating system are formed in the X-axis direction or the Y-axis direction. However, when the exposure process is carried out using the asymmetric illuminating system, the light passed through the asymmetric illuminating system has directionality based on the asymmetric shape of the illuminating system, thereby varying the process margin depending on the position of the patterns. Especially, since the dipole illuminating system has different resolution depending on the orientation of patterns to be exposed, it has a high resolution with respect to the patterns having the same orientation as the aperture of the illuminating system. On the contrary, the dipole illuminating system has a relatively degraded resolution compared with the symmetric illuminating system with respect to the patterns having orientation perpendicular to the apertures of the illuminating system.
In fact, the patterns having orientation perpendicular to the dipole illuminating system may generate defects such as collapsing or bridging of patterns after carrying out the exposure process. Furthermore, in the layers using the dipole illuminating system, the size of the pattern layout design varies depending on the orientation of the illuminating system. Therefore, a method for improving resolution and stability with respect to the patterns having orientation perpendicular to the illuminating system is demanded.